Guide to Plant-PET Imaging Using 11CO2.

Due to its high sensitivity and specificity for tumor detection, positron emission tomography (PET) has become a standard and widely used molecular imaging technique. Given the popularity of PET, both clinically and preclinically, its use has been extended to study plants. However, only a limited number of research groups worldwide report PET-based studies, while we believe that this technique has much more potential and could contribute extensively to plant science. The limited application of PET may be related to the complexity of putting together methodological developments from multiple disciplines, such as radio-pharmacology, physics, mathematics and engineering, which may form an obstacle for some research groups. By means of this manuscript, we want to encourage researchers to study plants using PET. The main goal is to provide a clear description on how to design and execute PET scans, process the resulting data and fully explore its potential by quantification via compartmental modeling. The different steps that need to be taken will be discussed as well as the related challenges. Hereby, the main focus will be on, although not limited to, tracing 11CO2 to study plant carbon dynamics.

Studying in vivo dynamics of xylem-transported 11CO2 using positron emission tomography.

Respired CO2 in woody tissues can build up in the xylem and dissolve in the sap solution to be transported through the plant. From the sap, a fraction of the CO2 can either be radially diffuse to the atmosphere or be assimilated in chloroplasts present in woody tissues. These processes occur simultaneously in stems and branches, making it difficult to study their specific dynamics. Therefore, an 11C-enriched aqueous solution was administered to young branches of Populus tremula L., which were subsequently imaged by positron emission tomography (PET). This approach allows in vivo visualization of the internal movement of CO2 inside branches at high spatial and temporal resolution, and enables direct measurement of the transport speed of xylem-transported CO2 (vCO2). Through compartmental modeling of the dynamic data obtained from the PET images, we (i) quantified vCO2 and (ii) proposed a new method to assess the fate of xylem-transported 11CO2 within the branches. It was found that a fraction of 0.49 min-1 of CO2 present in the xylem was transported upwards. A fraction of 0.38 min-1 diffused radially from the sap to the surrounding parenchyma and apoplastic spaces (CO2,PA) to be assimilated by woody tissue photosynthesis. Another 0.12 min-1 of the xylem-transported CO2 diffused to the atmosphere via efflux. The remaining CO2 (i.e., 0.01 min-1) was stored as CO2,PA, representing the build-up within parenchyma and apoplastic spaces to be assimilated or directed to the atmosphere. Here, we demonstrate the outstanding potential of 11CO2-based plant-PET in combination with compartmental modeling to advance our understanding of internal CO2 movement and the respiratory physiology within woody tissues.

Plant-PET to investigate phloem vulnerability to drought in Populus tremula under changing climate regimes.

Phloem transport is of great importance in trees to distribute assimilated carbon across the entire tree. Nevertheless, knowledge of phloem is incomplete, because of the complexity of measuring its transport and characteristics. Only few studies have addressed how phloem transport might alter under climatic changes, with most data originating from theoretical studies. We measured phloem characteristics in leaves of young Populus tremula L. trees grown during 5 months under ambient (TA, 404 ppm ± 5) and elevated (TE, 659 ppm ± 3) atmospheric CO2 concentration ([CO2]) using a combination of positron emission tomography (PET) and compartmental modelling. Short-term phloem dynamics were measured in vivo and non-invasively using the short-lived isotope of carbon, 11C (half-life 20.4 min). Trees were scanned in well-watered and dry conditions to assess changes in phloem characteristics induced by drought. Reliability of the PET-derived results was verified with reported observations in the literature. Phloem speed was highest in well-watered TE trees and strongly reduced by 81% under drought, whereas phloem speed reduced by 61% in TA trees at the same level of drought. These findings led us to speculate that phloem transport in TE trees might be more vulnerable to drought. We discuss how a higher phloem vulnerability to drought in a changing climate could impact tree hydraulic functioning. Taken together our results suggest that trees grown for 5 months under elevated [CO2] seem to be less well-acclimated to face projected hotter droughts in a changing climate.